The present invention relates to the field of power supplies, and in particular, to checking the charge state of a rechargeable power-supply. The present invention is especially, but not exclusively, applicable to determining at any instant the remaining operational time of a rechargeable power-supply in a cellular mobile telephone.
To check the charge state of a battery, and consequently, to be able to possibly determine the remaining operational time of this battery, the amount of charge passing through a resistor connected in series with the battery is measured. One approach currently used includes connecting a voltage-frequency converter to the terminals of the resistor. However, so that the energy consumed in the resistor is as small as possible so as not to cause problems in measuring the amount of charge, it is necessary to have a low enough resistance.
This then requires the use of a high-accuracy converter. However, even in this case, the offset voltage of the converter may prove to be problematic and lead to false information. Moreover, a converter of this type is sensitive to the supply voltage. The result of this is, in particular, a variation in the gain of the converter with a variation in the supply voltage.
Another approach includes using a conventional converter of the delta-sigma type. A converter of this type is found to be very accurate provided a high enough sampling frequency is used. However, this contributes to an increase in the current consumption of the apparatus. To overcome this drawback, it is possible to reduce the sampling frequency, but at the risk of then losing information.
In view of the foregoing background, an object of the present invention is to check the charge state of a rechargeable power-supply while minimizing any loss of information, and while doing so with low current consumption.
Another object of the present invention is to provide a checking device of this type which overcomes variations in the supply voltage.
These and other objects, advantages and features according to the present invention are provided by a method of checking the charge state of power-supply, and in particular, a rechargeable battery, by measuring an amount of charge passing through a resistor connected in series with the power-supply.
According to a general characteristic of the invention, the measurement of the amount of charge may comprise:
an analog integration over a predetermined integration time of an overall current equal to the sum of a current called a resistor current representative of that passing through the resistor, and of a predetermined reference current capable of selectively taking two opposite values of absolute value greater than the value of the resistor current;
successive comparisons throughout the integration time of the corresponding successive results of the integration with a comparison reference, for example, a zero voltage;
successive selections of one of the two opposite values of the reference current depending on the successive results of the comparisons; and
a count over the integration time of the number of times where one of the two opposite values was selected, for example, the number of times where the positive value of the reference current was selected, and the result of this count is representative of the amount of charge.
According to one embodiment of the method, the counting step involves the incrementation of a counter each time the value in question is selected (for example, the positive value of the reference current) and the non-incrementation of the counter each time the opposite value (for example, the negative value of the reference current) is selected. Half the value of the integration time is then subtracted from the counter value. The result of this subtraction furnishes a value representative of the amount of charge.
The result of this subtraction can be obtained simply by taking the two""s complement of the contents of the counter. This two""s complement is obtained by inverting the binary value of the significant bit of the counter value.
According to one embodiment of the method applicable to a differential measurement of the current passing through the resistor, the integration phase involves reversing the polarity of the differential measurement at half the integration time and continuing the integration for the second half of the integration time with the polarity inverted in this way. Moreover, during the polarity reversal, the values of the results of the comparisons are also reversed. This embodiment makes it possible, when the integration is carried out by an operational amplifier, to compensate for the offset voltage of this operational amplifier.
It is also possible to determine the remaining operational time of the power-supply at a given instant from the accumulation at this instant of the amount of charge measured during successive integration times, and from a calculation of the current consumption at this given instant. The result of the count over a predetermined number of previous integration times furnishes, for example, a calculation of this current consumption.
The subject of the invention is also a device for checking the charge state of a power-supply. The device comprises a resistor connected in series with the power-supply, and measurement means connected to the terminals of the resistor for measuring the amount of charge passing through the resistor.
According to a general characteristic of the invention, the measurement means or circuitry may comprise:
an input interface connected to the terminals of the resistor and delivering a resistor current;
a reference interface delivering a predetermined reference current capable of selectively taking two opposite values with an absolute value greater than the value of the resistor current;
provision means capable of providing a predetermined integration time;
analog integration means capable of carrying out an integration over the integration time, of an overall current equal to the sum of the resistor current and of the reference current;
comparison means capable of carrying out successive comparisons during the integration time of corresponding successive results of the integration with a comparison reference;
selection means capable of successively selecting one of the two opposite values of the reference current depending on the successive results of the comparisons; and
counting means capable of counting, over the integration time, the number of times when one of the two opposite values has been selected, the result of this count being representative of the amount of charge.
According to one embodiment of the invention, the integration means may comprise an operational amplifier fed back by a capacitor, receiving the overall current and carrying out a continuous analog integration of the overall current. The provision means may comprise a first counter which is incremented in time with a clock signal up to a maximum counting value corresponding to the integration time. The comparison means may comprise an analog comparator followed by a flip-flop regulated by the clock signal.
According to one embodiment of the invention, the counting means may comprise a second counter incremented each time the value in question is selected, and non-incremented each time the opposite value is selected. Moreover, the counting means may comprise means capable of subtracting half of the maximum counting value of the first counter from the value of the second counter. The result of this subtraction furnishes a value representative of the amount of charge.
The subtraction means may advantageously comprise an inverter connected to the output of the second counter delivering the significant bit. The first counter and the second counter are advantageously asynchronous counters, that is, they are counters whose internal flip-flops do not all switch at the same time. This makes it possible to obtain a very low consumption for these counters.
The device according to the invention advantageously may comprise an analog block and a digital block. The analog block may comprise the input interface, the reference interface, the integration means, and part of the comparison and selection means. The digital block may comprise the means for preparing the integration time, the other part of the comparison and selection means, and the counting means.
The analog block may advantageously have a differential structure which makes it possible to overcome stray capacitances, and consequently, effects of variations in the supply voltage. In a differential structure, the input interface comprises a differential input formed from two terminals. It is thus particularly advantageous that the measurement means furthermore comprise chopper means capable of inverting the two terminals at half the integration time. The integration continues during the second half of the integration time with the two terminals thus inverted. The chopper means also is capable of inverting the output signal of the comparator during the second half of the integration time. An embodiment of this sort makes it possible to compensate for the offset of the operational amplifier forming the integrator.
The chopper means may comprise, for example, an inverter connected to the two terminals of the input interface. This inverter is controllable by the value of the significant bit of the first counter, that is, the counter which determines the integration time.
Moreover, the chopper means may comprise an EXCLUSIVE OR logic gate. The logic gate includes a first input connected to the output of the significant bit of the first counter, a second input connected to the output of the comparison means, and an output connected to an input of the second counter controlling the incrementation or the non-incrementation of this second counter. This EXCLUSIVE OR logic gate makes it possible to invert the output of the comparator in the second half of the integration time.
To calculate the remaining operational time of the power-supply means, the device may advantageously comprise an accumulation register capable of accumulating the successive count results. Means then calculate the remaining operational time of the power-supply means from the contents of the accumulation register, and from a predetermined number of previous count results.
The subject of the invention is also an apparatus operating with a rechargeable power-supply. In particular, the apparatus may be a cellular mobile telephone incorporating a charge-checking device as defined above.